Electrical motor heating systems are known to the prior art for heating an electrical motor during the time that the electrical motor is shut down. The purpose of such heating is to minimize or to prevent condensation from occurring in the windings of the electrical motor, which condensation lowers the insulation resistance of the motor windings. If the condensation is not removed prior to the time that the motor is restarted, the lowered insulation resistance will, in most cases, result in damage to or destruction of the motor when the motor windings are re-energized.
Therefore, electrical motors, and particularly those which are located in industrial environments in which ambient temperatures may change rapidly, must be heated during shutdown to prevent substantial condensation from occurring. In the past, a source of heat, such as that provided by an electrical radiant heater, or even a light bulb, has been placed in proximity to the motor during the period of motor shutdown. There is practically no way, however, to insure that such methods will actually prevent condensation from occurring due to the uncertain nature of heat transfer between the source of heat and the motor windings. Motor damage or destruction often occurs upon the restarting of such motors unless a technician is sent out to measure the insulation resistance of the motor windings prior to restarting. As can be appreciated, such methods of heating are also expensive to implement, requiring the services of a trained individual who must connect and disconnect the source of heat and measure the insulation resistance of the motor if necessary.
The prior art has taught a number of methods and apparatus for automatically and more efficiently heating the motor windings upon motor shutdown. As taught in the patent to Crawford, U.S. Pat. No. 2,240,207, the secondary winding of a single-phase transformer, whose primary winding is interconnected with a suitable source of electrical power, is selectively interconnected with two motor winding terminals of a multi-phase motor only when the contacts of a main contactor supplying three-phase power to the motor winding are opened.
In the Crawford patent, a relay coil is actuated when auxiliary contacts associated with the main contactor are closed, with the auxiliary contacts being closed only when the main contacts thereof are opened. The contacts of this relay coil complete a circuit between the transformer secondary and the two terminals of the multi-phase motor.
As a result, a small voltage is applied to the motor winding during motor shutdown which results in a single-phase alternating current flowing through the motor winding. The magnitude of the alternating current is chosen to result in a temperature rise in the motor of a few degrees above the ambient temperature.
Although simple in concept and in execution, the method and apparatus taught in the Crawford patent has proved to encounter significant difficulties in practical applications. For example, the transformer and associated relay are cumbersome and generally cannot be placed in an enclosure also containing a starter for the motor. More important, it has been discovered that application of a voltage to a single winding of a multi-phase AC motor will result in a tremendously high current through that winding when the voltage exceeds a predetermined value. In the case of a 575 VAC, three-phase induction motor, a voltage in excess of 28 volts applied to a single-phase winding thereof will result in the current through the winding quickly going to infinity as the iron in the motor is saturated. If this high current is permitted to exist for a predetermined period of time, the winding insulation will fail and the motor winding will burn out. Since the method and apparatus taught in the Crawford patent continuously applies an alternating current to the motor winding upon motor shutdown, it is particularly difficult in practice to select a desired value of the voltage applied to the motor winding which will obtain the desired heating effect and yet not result in a current through the motor winding capable of causing motor burn-out.
Another motor heating system of the prior art is that taught in the patent to Koch, U.S. Pat. No. 2,338,518, in which the secondary of a single-phase transformer is manually switched into circuit with the terminals of the exciting winding of a multi-phase motor in such a manner so as to result in a single-phase alternating current flowing in the same direction through all phases of the exciting winding to achieve motor heating.
Yet another motor heating system of the prior art is that taught in the patents to Blair, U.S. Pat. No. 3,445,743 and U.S. Pat. No. 3,582,712. A circuit is provided for interconnecting the secondary winding of a single-phase transformer with the terminals of a multi-phase motor so as to apply a single-phase alternating current to all phases of the motor winding for motor heating purposes. The circuit in the Blair patents operates to automatically interconnect the transformer secondary with the motor terminals at a predetermined time after the motor is shut down.
As with the method and apparatus taught in the Crawford patent, those in the patents to Koch and Blair are disadvantageous in practical application for the reasons already noted.
A more recent approach in the prior art is that typified by the patents to Dikinis et al., U.S. Pat. No. 3,717,804, and Hann, U.S. Pat. No. 3,774,096. In the Dikinis and Hann patents, a plurality of contact pairs of a motor contactor interconnect a multi-phase power supply with a multi-phase motor. A first conductor shunts a first one of the contact pairs, and a motor heater circuit shunts a second one of the contact pairs. Within the motor heater circuit is located the series connection of current-carrying electrodes of a controlled semiconductor switch, such as an SCR or a TRIAC, and a fuse. Conduction of the controlled semiconductor switch is afforded by a timing circuit including a capacitor and a variable resistor also connected in series across the second one of the contact pairs with the common junction of the capacitor and the resistor being coupled to a gate electrode of the controlled semiconductor switch through a diode and a voltage breakdown device. In operation, the motor heater circuit is shunted by the second one of the contact pairs when the motor is in operation. When the contact pairs are opened, however, the timing circuit within the motor heating circuit functions to apply a gating pulse to the controlled semiconductor switch at a predetermined time during each half-cycle of the applied voltage. In response, the controlled semiconductor switch conducts for the remainder of each half-cycle to provide a current path between the current-carrying electrodes, and therefore through the fuse, the motor winding and the conductor shunting the first one of the contact pairs, so that a short current pulse is applied to the motor winding for a portion of each half-cycle to heat the motor.
The circuits in the Dikinis and Hann patents generate a significant amount of radio frequency interference inasmuch as the controlled semiconductor switches therein are switched on at a time when the applied voltage has a relatively high value.
More importantly, the circuits in the Dikinis and Hann patents are constrained to place the controlled semiconductor switches in a conducting condition during each half-cycle, or, in some cases, during each alternative half-cycle, of the applied voltage wave form. Assuming that a 575 VAC multi-phase power supply is used, it is apparent that the instantaneous voltage applied to the motor winding energized by the circuits in the Dikinis and Hann patents may exceed 28 volts, as previously discussed, during each half-cycle of the applied voltage wave form. Although the time duration of each current pulse provided by the circuits in the Dikinis and Hann patents is not sufficient to result in motor burn-out, the repeated application of such current pulses in successive half-cycles will most certainly result in motor burn-out, unless each pulse is provided for only a very short portion of each half-cycle. But, if the pulse time duration during each half-cycle is reduced to this amount, the requisite motor heating effect is often not obtained.
Finally, the use of R-C timing circuits in the Dikinis and Hann patents (to determine the amount of delayed conduction or "phase-control" therein) provides a disadvantage inasmuch as the time constants of such R-C circuits will change over an extended period of time. Therefore, the circuits in the Dikinis and Hann patents require, in actual application, continued re-examination by skilled personnel in which the amount of phase-control being provided is monitored and corrected if necessary. If such correction is not provided, then there is a possibility that the circuits in the Dikinis and Hann patents will apply longer current pulses than those initially applied to the motor winding, with a consequent probability of motor burn-out.
It will also be apparent to those skilled in the art that the aforementioned problems of using a motor winding to heat an electrical motor are also found in more generalized situations in which a coil is used in an induction heater. For example, it is oftentimes difficult in such situations to obtain a desired heating effect from the induction heater without also risking unduly high current flows through the coil of the induction heater and a consequent probability of coil failure.
It is therefore an object of this invention to provide an apparatus for heating an electrical motor during motor shutdown which is reliable in operation, even when left unattended for long periods of time.
It is a further object of this invention to provide such an apparatus which does not require the attention of skilled personnel at the time of installation, or at any subsequent time during motor shutdown, or when the motor is restarted.
It is yet a further object of this invention to provide such an apparatus which is simple and inexpensive to construct.
It is another object of this invention to provide such an apparatus which utilizes solid-state elements and which therefore can be packaged so as to be installed in an enclosure also containing a starter for the electrical motor.
It is yet another object of this invention to provide a method and apparatus for heating an electrical motor during shutdown which generates little or no radio frequency interference during operation.
It is still another object of this invention to provide a method and apparatus which achieves a maximum heating effect without the probability of coil or winding burn-out.